1. Field of the Invention
The present invention relates to a polarizing-plate-protecting film that can be placed on a polarizing film, for its protection, to form a polarizing plate, and to a polarizing plate using the polarizing-plate-protecting film.
2. Background Art
Polarizing plates are generally known for their use in liquid crystal displays (LCDs), etc. They usually use, as polarizers responsible for their property of polarizing light, polyvinyl alcoholic polarizing films, such as polyvinyl alcohol film containing adsorbed and aligned iodine. However, the polarizing films are poor in mechanical strength, so that polarizing-plate-protecting films are laminated to each side of the polarizing film with water-based adhesive layers. For the polarizing-plate-protecting films, triacetyl cellulose film is often used.
The surface of triacetyl cellulose film is poor in mar resistance. When the triacetyl cellulose film surface requires higher mar resistance, a hard coat layer is sometimes formed on the film surface by an ultraviolet-curable resin or the like (Patent Documents 1 and 2).
Although triacetyl cellulose film is excellent in optical characteristics, etc., it is at a disadvantage in that its production cost is relatively high. In order to overcome these disadvantages, it has been proposed, as a polarizing-plate-protecting film to be placed on the observation side of a polarizer, a biaxially oriented polyester film whose surface to be bonded to the polarizer is provided with a hydrophilic adhesion-facilitating layer of a polyvinyl alcoholic resin so that the polyester film can be easily bonded to the polarizer with a water-based adhesive layer. It has also been proposed that a hard coat layer, which is a hardened layer of a curable resin, or the like be further formed on the front surface of the polarizing-plate-protecting film, if necessary (Patent Document 3).
Further, it has been proposed that a special water-based adhesive such as a water-dispersible isocyanate adhesive or a crosslinkable, acetoacetyl-group-containing polyvinyl alcoholic adhesive be used to form a water-based adhesive layer with which a polarizing-plate-protecting film composed of triacetyl cellulose film, a polyester film, or the like and a polyvinyl alcoholic polarizing film are laminated (Patent Documents 4 and 5).
Patent Document 1: Japanese Patent Publication No. 065095/1992
Patent Document 2: Japanese Laid-Open Patent Publication No. 157791/1994
Patent Document 3: Japanese Laid-Open Patent Publication No. 271733/1996
Patent Document 4: Japanese Laid-Open Patent Publication No. 107245/2003
Patent Document 5: Japanese Laid-Open Patent Publication No. 010760/2005
Liquid crystal displays having polarizing plates have so far been used mainly for watches, gauge panels, and the like whose display surfaces have relatively small areas or are provided with reflection resistance of light diffusion type. In recent years, however, they have come to be used for large-screen televisions, etc. whose display areas are large or whose surfaces are provided with reflection resistance of multi-layer interference type so that the displays have glossy surfaces having low haze values. Such liquid crystal displays have the shortcoming that fringes and wavelike patterns (sometimes referred collectively to as “tatamime” (meshes-of-tatami-mat-like patterns)), scratches, stains, and so forth on the surfaces of polarizing-plate-protecting films are noticeable, so that to overcome this shortcoming has become practical necessity.
A major factor in the above-described shortcoming is that those polarizing-plate-protecting films that are now mainly used in liquid crystal displays, etc. use triacetyl cellulose film. The stain resistance of the polarizing-plate-protecting film can be improved to some extent if the film surface is provided with a hard coat layer by the use of a curable resin or the like. However, there has been a limit to improvement in mar resistance, particularly in surface toughness that is evaluated by pencil hardness, because the polarizing-plate-protecting film itself, the underlying layer of the hard coat layer, is soft. Moreover, it has been impossible for the hard coat layer to eliminate fringes and wavelike patterns that appear on the polarizing-plate-protecting film.
The shortcoming originating from triacetyl cellulose film itself can be overcome by using, as the polarizing-plate-protecting film, a polyester film such as polyethylene terephthalate film. However, the surface of a polyester film is less hydrophilic than triacetyl cellulose film, so that a polyester film cannot be firmly bonded to a polyvinyl alcoholic polarizing film with a water-based adhesive that is usually used for bonding to a polyvinyl alcoholic polarizing film. Even when the special adhesives disclosed in the above-described Patent Documents 4 and 5 are used, sufficiently high adhesion cannot be obtained between a polyester film and a polyvinyl alcoholic polarizing film, and, moreover, other special adhesive is needed to bond the two films. If a hydrophilic adhesion-facilitating layer is formed beforehand by a polyvinyl alcoholic resin on the bonding surface of a polyester film, as described in Patent Document 3, a conventional water-based adhesive can be used to bond the polyester film to a polyvinyl alcoholic polarizing film, and good adhesion can be obtained between the two films. However, since the adhesion of the hydrophilic adhesion-facilitating layer to the polyester film is poor, satisfactory adhesion cannot be obtained as a whole. Moreover, since the adhesion-facilitating layer is a hydrophilic vinyl alcoholic layer, the existing coating equipment for applying a coating diluted in an organic solvent, which is now widely used to form an adhesion-facilitating layer, cannot be used. In addition, to use the existing coating equipment, modification of the equipment, such as increasing the dryer capacity and making the equipment corrosion proof, or design change is necessary.
Further, the surface toughness required for a polarizing-plate-protecting film includes light resistance in addition to mar (scratch) resistance and stain resistance. It has been difficult to provide a polarizing-plate-protecting film with all of these properties. Namely, a typical means for providing a polarizing-plate-protecting film with mar resistance and stain resistance has been the use of a hard coat layer that is formed by cross-linking, with ultraviolet light, an ultraviolet curable resin containing acrylate monomer or the like. A typical means for providing a polarizing-plate-protecting film with light resistance has been the addition of an ultraviolet light absorber such as benzotriazole. However, it has been extremely difficult to use the two means at the same time. This is because if an ultraviolet light absorber is added to an ultraviolet curable resin, the resin unfavorably absorbs ultraviolet light applied to cure the resin.